This relates generally to electronic devices, and more particularly, to electronic devices with wireless communications capabilities.
Electronic devices with wireless communications capabilities typically include amplifying circuits that are used to amplify the power of radio-frequency signals prior to wireless transmission. For example, a radio-frequency power amplifier may receive input signals having an input power level and generate corresponding output signals having an output power level, where the output power level of the output signal is generally greater than the input power level of the input signal. Ideally, the power amplifier exhibits a perfectly linear input-output power transfer characteristic (i.e., an increase in the input power by a certain amount should result in a corresponding predetermined amount of increase in the output power).
In practice, however, power amplifiers often exhibit non-linear behavior. When a power amplifier is non-linear, an increase in the input power may result in a corresponding increase in the output power that is different than the predetermined amount. Amplifier non-linearity issues can degrade signal integrity and adversely impact wireless performance.
Consumer electronic devices are sometimes configured to support complex, non-constant envelope modulation schemes such as Wideband Code Division Multiple Access (W-CDMA) and Long Term Evolution (LTE) that encode digital data using Orthogonal Frequency-Division Multiplexing (OFDM). High frequency signals generated using such types of radio access technologies can exhibit high peak-to-average ratios (PARs), which places stringent requirements on the linearity of the power amplifier. This increases the power consumption of the power amplifier, which negatively impacts battery life. In order to improve the battery life, it is generally desirable to operate the power amplifiers in the non-linear region.
When radio-frequency power amplifiers are operated in the non-linear region, however, undesired spectral regrowth may be generated that degrades the transmit modulation quality. To reduce this effect, predistortion calibration operations are typically performed to linearize the wireless system. Predistortion calibration involves steps for obtaining amplitude and phase coefficient terms that are used to predistort signals in the modem, which are fed to the transceiver for digital to high frequency RF conversion. This ensures satisfactory transmit quality without compromising on efficiency.
Conventional predistortion calibration, however, requires capturing IQ samples at the output of the power amplifier, correlating the captured output IQ samples to input IQ samples, and then computing the inverse to obtain predistortion coefficients that linearize the system. This step has to be performed at each channel within each band of interest. Performing predistortion calibration for every in-band channel in this way can be extremely time consuming and costly to implement.
It would therefore be desirable to provide improved ways for computing predistortion coefficient values for wireless systems.